Cytochrome p450s catalyze enzymatic reactions for a diverse range of chemically dissimilar substrates that include the oxidative, peroxidative and reductive metabolism of endogenous and xenobiotic substrates. In plants, p450s participate in biochemical pathways that include the synthesis of plant products such as phenylpropanoids, alkaloids, terpenoids, lipids, cyanogenic glycosides, and glucosinolates (Chappel, Annu Rev. Plant Physiol. Plant Mol. Biol. 198, 49:311-343). Cytochrome p450s, also known as p450 heme-thiolate proteins, usually act as terminal oxidases in multi-component electron transfer chains, called p450-containing monooxygenase systems. Specific reactions catalyzed include demethylation, hydroxylation, epoxidation, N-oxidation, sulfooxidation, N-, S-, and O-dealkylations, desulfation, deamination, and reduction of azo, nitro, and N-oxide groups.
The diverse role of Nicotiana plant p450 enzymes has been implicated in effecting a variety of plant metabolites such as phenylpropanoids, alkaloids, terpenoids, lipids, cyanogenic glycosides, glucosinolates and a host of other chemical entities. During recent years, it is becoming apparent that some p450 enzymes can impact the composition of plant metabolites in plants. For example, it has been long desired to improve the flavor and aroma of certain plants by altering its profile of selected fatty acids through breeding; however very little is known about mechanisms involved in controlling the levels of these leaf constituents. The down regulation of p450 enzymes associated with the modification of fatty acids may facilitate accumulation of desired fatty acids that provide more preferred leaf phenotypic qualities. The function of p450 enzymes and their broadening roles in plant constituents is still being discovered. For instance, a special class of p450 enzymes was found to catalyze the breakdown of fatty acid into volatile C6- and C9-aldehydes and -alcohols that are major contributors of “fresh green” odor of fruits and vegetables. The level of other novel targeted p450s may be altered to enhance the qualities of leaf constituents by modifying lipid composition and related break down metabolites in Nicotiana leaf. Several of these constituents in leaf are affected by senescence that stimulates the maturation of leaf quality properties. Still other reports have shown that p450s enzymes are play a functional role in altering fatty acids that are involved in plant-pathogen interactions and disease resistance.
In other instances, p450 enzymes have been suggested to be involved in alkaloid biosynthesis. Nornicotine is a minor alkaloid found in Nicotiana tabaceum. It has been postulated that it is produced by the p450 mediated demethylation of nicotine followed by acylation and nitrosation at the N position thereby producing a series of N-acylnonicotines and N-nitrosonornicotines. N-demethylation, catalyzed by a putative p450 demethylase, is thought to be a primary source of nornicotine biosyntheses in Nicotiana. While the enzyme is believed to be microsomal, thus far a nicotine demethylase enzyme has not been successfully purified, nor have the genes involved been isolated.
Furthermore, it is hypothesized but not proven that the activity of p450 enzymes is genetically controlled and also strongly influenced by environment factors. For example, the demethylation of nicotine in Nicotiana is thought to increase substantially when the plants reach a mature stage. Furthermore, it is hypothesized yet not proven that the demethylase gene contains a transposable element that can inhibit translation of RNA when present.
The large multiplicity of p450 enzyme forms, their differing structure and function have made their research on Nicotiana p450 enzymes very difficult before the enclosed invention. In addition, cloning of p450 enzymes has been hampered at least in part because these membrane-localized proteins are typically present in low abundance and often unstable to purification. Hence, a need exists for the identification of p450 enzymes in plants and the nucleic acid sequences associated with those p450 enzymes. In particular, only a few cytochrome p450 proteins have been reported in Nicotiana. The inventions described herein entail the discovery of a substantial number of cytochrome p450 fragments that correspond to several groups of p450 species based on their sequence identity.